1. Field of the Invention
This invention relates generally to electro-optical measurement of the physical properties of materials whose particulate characteristics may be classified as being large and irregular in shape and with significant fractions of mass concentration associated with fibrous or flaky material having at least one dimension much larger than 3 .mu.m. Cotton dust is a material which falls into this classification and, this invention relates particularly to the continuous in situ electro-optical weighing of cotton dust.
2. Description of the Prior Art
Reference may be made to U.S. Pat. Nos. 3,797,937; 4,017,186; both of which relate to electro-optical measurement techniques. U.S. Pat. No. 3,797,937 describes a system for making particle measurements by forward scattering of laser light. U.S. Pat. No. 4,017,186 describes an electrical-optical method and system for in situ measurements of particulate mass density using back scattering techniques. The latter patent is concerned only with measurements of particulates which fall in the range of 0.3.lambda. to 3.lambda..
Overwhelming scientific evidence supports the finding that cotton dust produces adverse health effects among cotton workers. The disorders range from an acute reaction manifested by a depression of pulmonary function indicators, or by subjective symptoms such as chest-tightness, shortness of breath, or cough, to a stage characteristic of chronic obstructive pulmonary disease which is often disabling. The chronic stages of cotton dust induced respiratory disease are, as a clinical entity, similar to chronic bronchitis or emphysema. While gaps exist in the understanding of the etiology of respiratory disease caused by cotton dust and their progression from acute to chronic stages, evidence supports the fundamental connection between cotton dust and various respiratory disorders in both the textile and the non-textile industries.
Because of the recognized health dangers of cotton dust, the Department of Labor through the Occupational Safety and Health Administration (OSHA) has adopted occupational safety and health standards related to occupational exposure to dust. Monitoring of exposure levels and dust sampling is of increasing importance to the cotton industry.
Cotton dust ranges in size from particles large enough to be visible to the naked eye to those which are submicron in size. The shape of the particles is also irregular. Therefore, particle size is equated with the "aerodynamic equivalent diameter," i.e. the size of a unit density sphere having the same settling velocity as the particle in question, of whatever size, shape or density. Most investigators believe that a better correlation exists between respirable dust and health effects, since total dust measurements include a significant fraction of particles which are too large to be deposited in the respiratory tract, but the question has not been settled conclusively. Early studies have involved the use of the vertical elutriator for measuring employee exposure to cotton dust. These instruments have a theoretical cutoff of 15 microns. While this method collects somewhat more than the respirable fraction, including particles larger than approximately 15 microns in size, exposure data derived from the use of the vertical elutriator is primary from smaller dust particulates; little information is available on the layer particles normally considered too large to be deposited deep in the respiratory tract. Nevertheless, the vertical elutriator has been accepted in the industry as a valuable monitoring tool.
The vertical elutriator operates on the principle that particles with settling velocities less than the velocity of an air stream will be carried upward by a stream of air in a cylinder. The flow rate of 7.4 liters/minute is required to achieve cut off size at 15 .mu.m. The larger particles, with settling velocities greater than that of the vertical air stream, will settle out during their course of upward motion and will not be measured, whereas the smaller particles reach the top and are collected on a filter.
The acceptance of the vertical elutriator in the industry, both for research and periodic monitoring purposes, does not diminish its attendant disadvantages. The vertical elutriator, including pump, is approximately three feet in height, six inches in diameter and weights approximately 15 lbs.; therefore, its use at its current stage of development generally requires fixed sampling sites. Further the particles in its effluent which are collected on the filter must thereafter be measured by gravimetric techniques, and as hereinbefore noted, the effluent tends to include larger particles. although The presence of lint type particulates on filters diminishes the validity of such measurements for medical purposes.
Determining exposure to cotton dust in a large plant may require many samples. Since several hours are required for each sample, it is obvious that to complete the sampling, several days may be required and a number of units must be used simultaneously. Set up and labor costs are thus high. Other problems in using the vertical elutriator are: the necessity of relatively regular maintenance for its motors; the time consumed in the process of pre- and post-sample weighing of filters; the difficulty of calibration and maintaining calibrated flow rates; and, because of their size, the susceptiblity of samplers to damage during transportation.
Other sampling devices have been utilized for collecting respirable cotton dust. A GCA dust monitor with miniature vertical elutriator attachment utilizes the principle of a radioactive source to determine the amount of impacted dust. At higher dust levels (generally about 700 .mu.g/m.sup.3), its correlation with the vertical elutriator results is erratic.
The use of the horizontal elutriator and Hexhlet has been tried by some investigators as has a cyclone apparatus attached to a high volume sampler; however, this apparatus is even more bulky, expensive, and complicated than the vertical elutriator.
While the vertical elutriator (VE) has evolved as the primary sampling device in the textile industry and has served the textile industry well, it is generally recognized that alternative measurement methods could, desirably, reduce the cost of cotton dust measurements and, hopefully, increase their precision and accuracy. Many have commented on VE performance and data interpretation. Some have suggested that improved means can and ought to be used for the measurement of cotton dust. Such comments and question arise primarily because the actual VE penetration efficiency is not known.
Penetration efficiency F(d.sub.i) is defined as ##EQU1## The interval (d.sub.i, .DELTA.d.sub.i) describes the range of particles in the ith class and may be taken as d.sub.i -d.sub.i /2&lt;d&lt;d.sub.i +.DELTA.d.sub.i /2. .DELTA..chi. is the mass concentration (i.e. mass/unit volume of sampled air) and usually has dimensions of .mu.g/m.sup.3. The total mass concentration is .chi.=.SIGMA..DELTA..chi..sub.i. .chi. can obviously be computed for the collected sample or for the total workplace or environment air, provided the respective .DELTA..chi..sub.i 's are known from sizing or classifying measurements, such as cascade impactors or optical analyzers.
Total .chi.'s are also directly measureable. Historically, the simple, integrating measurements are performed first, before more difficult classifying measurements are made. The VE method yields .chi..sub.VE. An open-faced filter or total dust sample yields .chi..sub.T for the workplace. Particle size information can be generated by microscopic, optical or electron analysis of the filters.
The ideal and commonly understood-VE penetration efficiency is, by definition, ##EQU2## However, it is known that particles having aerodynamic equivalent diameters larger than 15 .mu.m can be and are collected. This is in addition to fibrous ("lint") or flakey particles frequently observed on the filters and commonly accepted as normal by cotton dust measurement practitioners. Lint particles can have lengths of several hundred micrometers.
Thus, the actual VE penetration efficiency F actual departs significantly from F ideal, especially for large particles. Despite the recognized failings of VE measurements, cotton dust sampling has followed the historical trend. Dust measurements with the VE and total dust sampler, which do not give mass fractions, were performed first and are far more common. Thus, the VE constitutes the basis for the cotton dust standard. Since size-resolved or size-classified measurements (i.e., .DELTA..chi..sub.i) are more difficult and expensive, they have in the past been generally only performed by specialists for research purposes, and are sparsely published. This partially explains why the actual penetration efficiency for the VE is poorly known; sufficient .OMEGA..chi..sub.i data are not readily available to permit unequivocal specification of F actual.